DIAGNOSIS: Mold or old food residue trapped under the removable silicone sealing ring on the underside of the lid—a major cleaning oversight.
FIX: Twist the lid to the unlock position and pull up on the center black silicone seal. It pops out. Wash the seal and the lid's groove with warm, soapy water. Use a toothbrush or bottle brush to scrub the groove thoroughly. Rinse and dry completely before snapping the seal back into place.
IF THAT DIDN'T WORK: For stubborn stains or mildew on the silicone seal, soak it for 30 minutes in a 50/50 mix of white vinegar and water, then scrub and rinse.
HEADS UP: Never put the lid with the seal still attached in the dishwasher. The high heat and jet spray can dislodge the seal, and food particles will get trapped behind it, making the problem worse. Always remove and clean the seal separately.